1. Field of the Invention
The present invention concerns an illumination system, particularly one that is used for lithography, for example, VUV and EUV-lithography with wavelengths of less than or equal to 193 nm, which illuminates a field, wherein the illumination system comprises at least one light source as well as optical elements, which are divided into raster elements.
2. Description of the Prior Art
In order to be able to even further reduce the structural widths for electronic components, particularly in the submicron range, it is necessary to reduce the wavelength of the light utilized for microlithography.
For example, lithography with soft x-rays is conceivable at wavelengths of smaller than 193 mn. A double-facetted illumination system for such wavelengths has become known, for example, from DE 199 03 807. The disclosure content of DE 199 03 807 is incorporated herein by reference.
In the case of the illumination system known from DE 199 03 807, first raster elements, which are also denoted field raster elements, are illuminated via collecting optics. When arranged in reflection, illumination is produced at a specific angle of incidence. Therefore, the illuminated field on the field raster elements is preferably elliptical. The field raster elements are configured rectangularly, however, corresponding to the desired field in the object plane, which coincides with the reticle plane.
Each field raster element is imaged in a field in an object plane, in which the reticle is positioned. Since each field raster element contributes to the uniformity of the illuminated field, field raster elements that are only partially illuminated adversely affect the uniformity in the object plane. Thus, only completely illuminated field raster elements should be used.
In systems, such as are known from DE 199 03 807, the first raster elements or facets have a typical aspect ratio of approximately 1:16. In such systems, field raster elements cover only about 80% of the area illuminated by the light source, i.e. approximately 20% of the power is lost. Since the number of field raster elements is limited by the optical elements arranged in the light path from the light source to the reticle plane behind the optical element with first raster elements and for reasons of geometry and capability of construction, a better efficiency for an illumination system as is known from DE 199 03 807 cannot be achieved simply by increasing the number of field raster elements.